Numerous particulate casting materials have been applied to form a glass cover and skin on steel continuously cast into open-top vertical casting molds with varying success in accomplishing the full range of desired functions of a protective flux-lubricant. Illustrative examples are disclosed in U.S. Pat. Nos. 3,052,936; 3,318,363; 3,642,052; 3,649,249; 3,677,325; 3,685,986; 3,704,744; 3,708,314; and 3,788,840. Such materials are variously and synonymously referred to as casting powder, flux powder, slag powder and agent or mixture for protecting or improving continuously cast steel. Typically these casting powders have been either particulate raw material mixtures dried of free water or preformed or prefused glasses or glassy substances which are comminuted to a mechanically flowable particulate form for distribution onto and over the surface of the molten steel in the upper part of the continuous casting mold.
Principal functions of a casting powder are preventing adverse heat loss from and oxidation of the molten steel at the open-top of the mold, removing deoxidation nonmetallic inclusions of alumina or silica from the molten steel by entrainment or dissolving in the fluid glass resulting from heating of the casting powder by the molten steel in contact therewith, and providing lubrication and heat transfer control between the solidified shell of the embryonic steel casting and the mold wall as the casting passes down through the mold. These functions ideally should be preformed in a manner to eliminate and prevent defects in the steel casting, such as entrapment or embedding of nonmetallic inclusions and formation of pin holes, surface cracks and mold oscillation marks. The degree to which a casting powder will successfully perform some or all of such functions greatly depends upon a proper balancing of a combination of characteristics, such as rapidity and uniformity of melting of the casting powder, viscosity characteristics of the resulting fluid glass covering from the time it is formed on the top surface of the molten steel in the mold on through the time it is to flow down between the mold wall and solidifying casting surface until it emerges from the bottom of the mold with and as a skin on the casting, and absence of formation of reactive substances from the casting powder in contact with the molten steel that produce defects that are retained in the steel as it solidifies. Rapid but nonuniform melting can lead to defects of unmelted casting powder particle deposits entrapped in the steel casting surface, which may later pop out to leave pin hole defects. It can also cause nonuniform viscosity through the fluid glass covering resulting in the formation of viscous stringers around the mold wall from which viscous globs separate without being uniformly absorbed into the rest of the fluid glass and either become deposits embedded in the surface of the steel casting or interfere with the proper flow of fluid glass covering between the casting surface and mold wall thereby contributing to poor heat transfer control and longitudinal cracking in the casting surface. Even rapid and uniform melting does not insure proper viscosity characteristics as the fluid glass coated steel casting cools while flowing over to the mold wall and moving downward in the mold. If a properly melted glass covering undergoes too rapid rigidification or devitrification, it may develop viscous stringers and/or it may not provide sufficient lubrication and heat transfer control whereby the steel castings suffer defects of cracking and/or oscillation marks on their surfaces. A casting powder that melts too slowly can contribute unmelted particle deposits in the steel casting surface, yield adverse viscosity characteristics that lead to formation of detrimental viscous stringers, and cause inadequate covering for proper lubrication and heat transfer control. Also, a dry casting powder may still contain volatilizable substance (such as chemically combined or absorbed water or carbonate) that detrimentally reacts with the molten steel to cause pin hole and deposit inclusion defects in the casting surfaces. Commonly, casting powders are formed as a glass or prefused material in attempts to provide uniformity of performance characteristics and eliminate adverse effects of volatiles that could react with molten steel. However, we have discovered that, within analytical compositions employed in our invention disclosed herein, casting powders of preformed or prefused glass suffer the detriments of too slowly melting when applied to the surface of molten steel. On the other hand, dry mixtures of commercially available ingredients yielding the same analytical compositions suffer from nonuniform melting characteristics although tending to exhibit very rapid melting.